This invention relates to fused overcoupled monomode optical fiber couplers and in particular to a method of producing such couplers and to a pressure sensor produced thereby.
Fused monomode optical fiber couplers are typically produced by heating a pair of optical fibers placed in contact, and pulling the fibers, whereby they first fuse at the periphery of their cladding, then neck down to form conical tapers. While the cladding is reduced in diameter at the waist region, the core is also reduced to the point of virtual disappearance. As an optical signal is carried by one of the fibers into the waist region, it leaves the core and becomes carried by the cladding. Coupling occurs into the second fiber in the waist region where the fibers are fused and the optical signal reaches the cladding periphery.
As the fibers are stretched, expanding the length of the waist region, the light energy coupled into the second fiber increases to a maximum, and then decreases to a minimum, cycling between maxima and minima. This phenomenon is described in U.S. Pat. No. 4,591,372 dated May 27, 1986 and assigned to Standard Telephones and Cables Public Ltd. Co. The phenomenon provides means for tuning by means of the coupler, thereby facilitating their use to form multiplexers.
In that patent the technique is described by which a pair of optical fibers are twisted and are held between clamps. In order to increase the length of the coupling region, a microtorch is held at a location between the clamps, and the clamps are cycled back and forth so that a longer region of the fibers is heated than if the clamps were held stationary. The forward clamp is moved faster than the rearward clamp, in order to effect stretching. As the clamps change direction, the rearward clamp becomes the forward clamp, and it moves faster than the other clamp.
It may be seen from FIGS. 5 and 6 of that patent that approximately three cycles of tuning can be obtained using the disclosed system.
It is desirable, however, to form long couplers, to increase the dependence of the coupling ratio on wavelength, and to increase the sensitivity of the coupling ratio to environment parameters.